Rate Limiter

-- Lua script for atomicity
local key = KEYS[1]
local window = tonumber(ARGV[1]) -- window size in seconds
local limit = tonumber(ARGV[2]) -- request limit
local now = tonumber(ARGV[3]) -- current timestamp

-- Deleting old entries
redis.call('ZREMRANGEBYSCORE', key, 0, now - window)

-- We count current requests
local count = redis.call('ZCARD', key)

if count < limit then
    -- Add a new request
    redis.call('ZADD', key, now, now .. '-' .. math.random())
    redis.call('EXPIRE', key, window)
    return 1 -- allowed
else
    return 0 -- rejected
end